In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
In such a recording method in dye diffusion transfer system, it has been known that it is important to make the image-receiving sheet have high heat insulation and cushion characteristics in order to give a favorable image.
Thus, in some cases, a composite support using a biaxial oriented (stretched) polyolefin film containing microvoids was used as a base material for the image-receiving sheet to make the sheet have more heat insulation and cushion characteristics. However in this method, there was occasionally caused a problem that the image-receiving sheet was wrinkled or curled by shrinkage due to relaxation of the residual stress after stretching by the heat during printing or the heat during formation of the image-receiving layer.
As other known methods of making the image-receiving sheet show heat insulation and cushion properties, a method in which, for example, a foaming layer composed of a resin and a foaming agent (see, e.g., Japanese Patent No. 2541796) or a heat insulation layer made of a micro-capsular hollow polymer and an organic solvent-resistant polymer as principal components (see, e.g., Japanese Patent No. 3226167) each having high cushion characteristics is formed between the support and the receptor layer, is known. The methods have an advantage that it is possible to prevent the image-receiving sheet from wrinkling and curling that are often found in the method in which the composite support, because a heat-insulating layer can be formed on a support by coating according to the method. However, it is generally difficult to produce a uniform smooth image-receiving sheet often causing problems such as bad image-transfer, a white deletion, a rough surface, and a deficient contact with a protective layer. To solve the problems described above, a method in which a solution for forming an intermediate layer is coated on a sheet-shaped support and an image-receiving sheet is formed while pressing the coated face to a cast drum in forming an intermediate layer of a resin containing hollow polymers as the principal component on the sheet-shaped support, is disclosed (see, e.g., JP-A-5-8572 (“JP-A” means unexamined published Japanese patent application)). However, although such a method is effective in giving sufficient smoothness, it makes the production process more complicated and is thus disadvantageous from the viewpoint of productivity.
To solve the aforementioned problems, for example, a heat-sensitive transfer image-receiving sheet containing a heat insulation layer and a receptor layer disposed on a support wherein the heat insulation layer contains a hollow polymer having a content of 65% by mass or more, and wherein the heat insulation layer and a adjacent layer at the receptor layer side are formed by a simultaneous multi-coating, is disclosed (see JP-A-2006-88691). In this solution, a high filling factor is required to achieve a sufficient heat insulating effect, which results in a coating solution having a high content of the hollow polymer. When coating solutions having a different volume shrinkage rate from each other at the time of drying, such as a coating solution containing a bulky solid material having voids inside and a latex coating solution free from voids inside, are applied by a simultaneous multi-coating, the surface of a coated film cracks after drying because of the different volume shrinkage rate at the time of drying during the coarse of drying after coating. Consequently, there was a problem relating to the image defects such as a white deletion at a low density portion and a non-uniformity of density at a high density portion.
Beside, a heat-sensitive transfer image-receiving sheet containing a heat insulation layer and a receptor layer disposed on a support wherein the heat insulation layer contains a hollow polymer having a content of 40% by mass or more, and the receptor layer contains a vinyl chloride/vinyl acetate copolymer emulsion or a vinyl chloride/acrylic compound copolymer emulsion, is disclosed (see JP-A-2006-264092). In a method of producing the image-receiving sheet as proposed in the above publication, the heat insulation layer and the receptor layer are formed by a sequential coating. However, the heat insulation layer is composed of a hollow polymer having such a high filling factor that even voids formed among hollow polymer particles are also used in order to achieve a sufficiently heat insulating effect. Therefore, when a layer at the receptor layer side is applied on the heat insulation layer by the sequential coating, a coating solution of the layer at the receptor layer side penetrates to the voids formed among hollow polymer particles in the outermost surface of the heat insulation layer that functions most efficiently as a heat insulation layer, which results in a problem of reduction in the heat insulating effect.
From the aforementioned background, it has been earnestly desired to develop a heat-sensitive transfer image-receiving sheet capable of achieving high density print characteristics by making the best use of the voids among hollow polymer particles that is accomplished by the simultaneous multi-coating, and also capable of realizing an excellent smoothness free from the crazing by the simultaneous multi-coating of the coating solutions having a different volume shrinkage rate from each other. As a solution for eliminating the irregularity of the coating, a method of producing a thermal transfer sheet in which uniformity of the coating is achieved by controlling viscosity of the coating solution, is disclosed (see JP-A-2006-130810). However, this publication only describes an invention directed to a single layer, and there is no reference to both occurrence and solution of the crack owing to the simultaneous multi-coating.